This invention relates to a surface-mount thin-profile capacitor and a method of producing the same and, in particular, to a surface-mount thin-profile capacitor including a metal foil as a base material and a method of producing the same.
A surface-mount thin-profile capacitor of the type includes a metal foil, such as an aluminum foil, as a base material. The metal foil comprises a metal core and a pair of etched layers covering opposite surfaces of the metal core. The metal foil is coated with an oxide film produced by “forming (electrochemical anodic oxidation in an electrochemical process)”.
Referring to FIG. 1, an existing surface-mount thin-profile capacitor will be described. In the figure, only a left-hand side of the surface-mount thin-profile capacitor is shown. Although not shown in the figure, a right-hand side has a similar structure. In other words, the surface-mount thin-profile capacitor has a symmetrical structure. In the surface-mount thin-profile capacitor illustrated in the figure, a metal foil as a base material is an aluminum foil 10.
The aluminum foil 10 comprises an aluminum core 11 and a pair of etched layers 12 covering opposite surfaces of the aluminum core 11. The aluminum foil 10 has opposite end portions which serve as anodes, and opposite surfaces each of which has a center area provided with a cathode. The cathode comprises a graphite (Gr) layer 15 and a silver (Ag) layer 16.
The surface-mount thin-profile capacitor illustrated in the figure has a resist resin 13 formed at a boundary between each anode and the cathode (15, 16), and a conductive polymer layer 14 formed inside of and on a surface of each of the etched layers 12 at the center area of the aluminum foil 10. The conductive polymer layer 14 is formed by polymerization of a conductive polymer. On a surface of the conductive polymer layer 14, the cathode (15, 16) is formed.
Next referring to FIG. 2 in addition to FIG. 1, description will be made of a method of producing the existing surface-mount thin-profile capacitor.
At first, the aluminum foil 10 is prepared. The aluminum foil 10 has the aluminum core 11 and the etched layers 12 formed on the opposite surfaces of the aluminum core 11. The etched layers 12 have a number of very small pores. As well known in the art, the aluminum foil 10 is already subjected to forming. Herein, the term “forming” means production of a dielectric film (aluminum oxide film) (not shown) on each of the opposite surfaces of the aluminum foil 10 by electrochemical anodic oxidation. In detail, the dielectric film is also formed on an internal wall of each small pore in the etched layers 12.
Generally, the dielectric film covering each surface of the aluminum foil 10 is readily damaged. In order to repair a damaged portion of the dielectric film, the aluminum foil 10 is subjected to re-forming (aging) (step S11). The opposite end portions, i.e., left and right end portions (the left end portion alone is shown in the figure) of the aluminum foil 10 are used as the anodes of the capacitor. As will later be described, the cathode (15, 16) is formed at the center area of each of the opposite surfaces of the aluminum foil 10. At the boundary between each anode and the cathode (15, 16), the resist resin 13 is applied to the etched layer 12 of the aluminum foil 10 (step S12). Subsequently, the aluminum foil 10 is subjected to re-forming (step S13). Furthermore, re-forming is carried out prior to polymerization (step S14).
Thereafter, by polymerization of the conductive polymer, the conductive polymer layer 14 is formed at the center area of the aluminum foil 10 separated by the resist resin 13 from an anode-side area (step S15). In this process, a part of the conductive polymer penetrates into the etched layer 12 of the aluminum foil 10. In other words, the conductive polymer enters into the small pores of the etched layer 12. By repeating polymerization, the conductive polymer layer 14 is formed to a height substantially equal to that of the resist resin 13. As shown in FIG. 1, a part 14a of the conductive polymer layer 14 at each of left and right ends thereof (only the left end is illustrated in the figure) may sometimes climb up onto an upper surface of the resist resin 13 to be exposed.
On the surface of the conductive polymer layer 14, the Gr layer 15 is formed (step S16). Then, on a surface of the Gr layer 15, the Ag layer 16 is formed (step S17). A combination of the Gr layer 15 and the Ag layer 16 is used as the cathode of the capacitor. Thus, the existing surface-mount thin-profile capacitor is produced.
In the surface-mount thin-profile capacitor of the above-mentioned structure, a porous portion of the etched layer 12 can not completely be filled by the resist resin 13. Therefore, as depicted by arrows A and B in FIG. 1, the conductive polymer and oxygen easily pass through the porous portion of the etched layer 12 inside the resist resin 13. As a result, the existing surface-mount thin-profile capacitor has following disadvantages.
(1) Entry of oxygen promotes oxidation deterioration of the conductive polymer. Therefore, high-temperature reliability can not be assured.
(2) Penetration of the conductive polymer towards the anode causes short-circuiting and an increase in LC.
(3) Exposure of the part 14a of the conductive polymer layer 14 climbing up onto the upper surface of the resist resin 13 causes oxidation deterioration of the polymer.
Presumably, the above-mentioned disadvantages are caused by the following reason. The porous portion of the etched layer 12 of the aluminum foil 10 has very small pores. Therefore, the resist resin 13 can not completely be filled from the surface of the aluminum foil 10 to the depth of the etched layer 12.